Impaired glucose metabolism (gestational diabetes (GDM) and mild hyperglycemia) that occurs during pregnancy is associated with an increased risk for pregnancy complications and is also an early indication of long- term metabolic dysfunction leading to diabetes and cardiovascular disease. In the US, GDM is commonly diagnosed using a two-step screening and diagnostic approach. The International Association of Diabetes and Pregnancy Study Group (IADPSG) proposed a one-step diagnostic approach that broadens the definition of GDM by lowering the cutoff values to include women with milder forms of hyperglycemia, who would have screened normal under the current two-step approach. The goal of these recommendations is better identification of women at risk for pregnancy complications and long-term metabolic dysfunction, but it results in a significant increase in the prevalence of GDM. The NIH Gestational Diabetes Consensus Development Conference committee does not recommend changing from the current two-step screening/diagnostic approach to the IADPSG one-step diagnostic approach without trials demonstrating that increasing the number of women diagnosed as having GDM results in better outcomes. In this application, we aim to 1) conduct a real world randomized controlled trial (RCT) to determine differences in short-term perinatal health outcomes with GDM testing between the two predominant GDM screening approaches, and 2) prospectively follow the mothers to examine their metabolic risk profiles and the growth of their infants at one year postpartum. Based on a pilot feasibility study, we propose a single site blinded RCT of 920 pregnant women at 18-24 weeks' gestation between ages 18-45 years who have not been diagnosed with diabetes, with a singleton pregnancy. Participants will have a non-fasting 1 hour 50 gm glucose challenge test (GCT) performed between 24-28 weeks' gestation. Women with 50 gm GCT results < 200 mg/dL will be randomized to receive either a fasting 2 hour 75 gm oral glucose tolerance test (OGTT) or a 3 hour 100 gm OGTT. GDM will be diagnosed using the IADPSG criteria for women receiving the 75 gm OGTT and Carpenter-Coustan criteria for women receiving the 100 gm. Participants and their physicians will be informed of the diagnosis of GDM, but will be blinded to the specific test results and criteria used to make the diagnosis. Participants with GDM will receive treatment as determined by their primary physician or midwife, and all participants will continue routine prenatal care. Brief questionnaires will be used to assess participants' and physicians' views on GDM testing. Metabolic profiles for participants will be assessed at randomization and at a year postpartum. The primary outcome measure is large-for-gestational age fetal growth. The rationale for the proposed research is that this is a unique opportunity to compare the two methods. At the end of the study, we will know whether women diagnosed at lower glucose levels with the IADPSG criteria are more likely to have adverse perinatal outcomes. It is our hypothesis that using IADPSG diagnostic criteria will result in detection of more women with impaired glucose metabolism and that treating these women will reduce adverse perinatal outcomes and prevent long-term metabolic dysfunction. This study will provide data grounded in level A evidence, to evaluate the two screening methods so universal guidelines for GDM screening can be endorsed by major organizations and implemented into clinical care.